This research focuses on three aspects of choice behavior: (1) transitivity. Many theories of choice imply that if A is chosen over B more than 50% of the time, and B over C more than 50% of the time, A will be chosen over C more than 50% of the time. The present research tests transitivity in the choice behavior of pigeons, rats, and human subjects. (2) conditional probabilities of choice. Many theories describe choice as a function of the values of the alternatives, but it is known that choice may also vary as function of the organism's immediate history of choices. The present research focuses on two types of conditional probabilties: one based on the alternative chosen last, and one based on the time elapsed since the last choice response. (3) Accessibility of the alternatives in time. Studies of choice in pigeons have established that preference for A over B will be highest when the alternatives are immediately accessible, and will decrease as access to the alternatives is progressively delayed. Few models of choice can describe such changes in preference. The present research explores the range of conditions under which these accessibility effects prevail.